1. Field of the Invention
This invention relates to an apparatus for producing welded metallic can bodies, and particularly, to an apparatus comprising roll-like revolving electrodes and a linear elongate electrode for producing welded metallic can bodies by mash seam resistance welding.
More specifically, this invention pertains to an apparatus for producing welded metallic can bodies, which comprises two revolving electrodes and an elongate electrode, and in which the lap portions of can bodies are adapted to be disposed respectively between the two revolving electrodes and the elongate electrode, and an alternating current flowing in these lap portions is supplied by coupling the two revolving electrodes to a power supply or by dividing the elongate electrode into two electrically insulated portions and coupling them to the power supply.
The "revolving electrode", as used herein, denotes a type of electrode which rotates while moving in a predetermined direction. It should be understood therefore that the terms "revolving" as a qualifier and "revolve" as a verb are used in this sense in the present specification and the appended claims.
2. Description of the Prior Art
An apparatus for seam-welding the lap portion of a can body by inserting it between a pair of rotating roller electrodes disposed at fixed positions and applying an alternating current across the electrodes while moving the lap portion has been widely known for the production of welded metallic can bodies of cans for holding aerosols, beer, powdery coffee, etc. and 18-liter cans. An apparatus comprising a roll-like revolving electrode and a linear elongate electrode has also been known widely. It is also well known that to avoid wear of the electrodes, seam welding is carried out while a wire-like electrode is interposed between the lap portion of the can body and the electrode.
In a general apparatus for the production of a welded metallic can body using a revolving electrode and an elongate electrode, a can body is disposed so as to surround the elongate electrode provided fixedly, and a high-frequency voltage is applied across the elongate electrode and the revolving electrode. The revolving electrode moves with rotation. As a result, the lap portion of the can body is successively pressed by the elongate electrode and the revolving electrode, and a high-frequency current is passed through the lap portion, thereby welding the lap portion.
The conventional apparatus, however, has the following problems.
To increase the speed of production, it is necessary to increase the speed of welding, i.e. the revolving speed of the revolving electrode. For increasing the speed of welding and performing welding satisfactorily, it is necessary to increase the frequency of the current from an ac power supply to about 200 to 500 Hz. This in turn gives rise to the following problems. Firstly, flowing of the current becomes difficult. An ac current with an rms of 3 to 9 KA is required. This increase in frequency, however, results in difficulty in the flowing of the current. It is believed to be due mainly to the increase of the inductance L in the impedance Z. The magnitude of inductance L is markedly affected by the degree of opening of an electrical circuit on the secondary side of a transformer. The value of the inductance L becomes larger as the degree of opening of the electrical circuit becomes larger. For example, if the distance between two electrical paths from an ac power supply to the welding portion is large, the value of the inductance L becomes high. On the other hand, in the conventional apparatus described above, one of the electrical paths from the ac power supply to the welding portion is made up of a conductor leading up to the elongate electrode, and the other is a feeder leading to the revolving electrode. The revolving electrode moves relative to the elongate electrode. Accordingly, the two electrical paths cannot be caused to approach and the inductance L cannot be reduced. A second problem concerns the generation of heat at various parts. When a welding current having a relatively high frequency of about 200 to 500 Hz is used and, for example, a conductor exists in the secondary side circuit of the transformer, an eddy current occurs in the conductor to generate heat. This heat generation is particularly great in a magnetic material such as iron. In the prior art, therefore, it is necessary to limit the materials constituting various component parts, or a complex mechanism should be provided to cool these component parts.